In the cable making art there are generally two types of insulation and/or jacket material used in the fabrication of electrical wire and cable, thermosetting materials and thermoplastic materials. The application of thermoset materials as the insulation or jacket of an electrical wire or cable requires the use of vulcanization (curing) apparatus to cause the thermosetting reaction to occur. Until recently the most widely accepted technique of manufacturing extruded vulcanized type cables was to pass the conductor though a series of extruder heads and apply concentrically the semiconducting, and insulating compounds. After application of the semiconducting and insulating compounds, the cables are vulcanized (cured) under pressure in a saturated steam environment followed by cooling under pressure. In such a steam curing process, the insulated conductor is moved through the vulcanizer and exposed to pressurized (typically 250 psi) saturated steam followed by cooling under pressurized water (typically 250 psi). The thermosetting compound contains curing agents which are activated at the high temperatures found within the vulcanizer with the speed of the vulcanization reaction depending on the temperature within the vulcanizer (for 250 psi steam, approximately 210.degree. C.).
The steam curing process has been used in horizontal, vertical, slant and catenary installations. Long length curing pipes have been demonstrated to be preferred by those practicing the art of steam curing cables, this is so because polyethylene and ethylenepropylene rubber insulations characteristically have high thermal resistances and therefore, heavily insulated cables of the type used for high voltage operation take a long time to cure. In addition to the problems of low cure rates and corresponding low production rates achieved using steam cure processes, is the problem created by the required exposure of the cable insulation to high pressure steam which has been shown to penetrate the insulation and to create microscopic cavities or voids within the insulation. Such voids limit or even reduce the dielectric strength of the cured insulation. In order to eliminate these and other problems associated with steam curing cable insulation, a number of different systems have been proposed, among these systems are those disclosed and claimed in U.S. Pat. Nos.: 3,635,621; 3,868,463; 3,901,633; 4,043,722; 4,069,286 and 4,080,131. The most significant system uses high temperature and pressure inert gases to cure the insulation and a dry cooling process to cool the cured insulation. While the transition to dry-cure vulcanization systems has improved the electrical properties of the vulcanizate, it has been found that additional problems have been created by abandoning the use of steam as the vulcanization heat source. Chief among the new problems encountered when a dry-cure/dry-cool vulcanization system is used in all but a full catenary arrangement is an oscillation or vibration of the cable which occurs as the extruder-vulcanizer production rate is increased past a critical point for the particular equipment being used. This oscillation causes a ripple effect in the insulation being applied because the cable is mechanically unstable as it passes through the extrusion die and therefore incapable of following a constant path as it moves through the extruder die. This oscillation also causes the insulation to be abraded by contact with the inner wall of the vulcanization tube. It has been found that this phenomenon is agravated when a tandem extrusion process is used to apply both the insulation and insulation shield so that only one pass through the vulcanizer is required to cure both the insulation and insulation shield and conversely, its effects are mitigated if separate passes through the extrusion apparatus are used for the application of the vulcanization of the cable insulation on the first pass and the insulation shield on the second pass. In accordance with the present invention it has been determined that the cause of the oscillation/vibration problem was related to the extrusion sequence in that when tandem extrusion is not used certain organic by-products of the curing reaction are discharged by the cable insulation and condensed on the interior walls of the vulcanization apparatus.
When a tandem extrusion process is used to extrude the insulation shield over the insulation prior to vulcanization these same cure reaction by-products are trapped inside the insulation by the insulation shield. When the by-products were analyzed they were found to contain about 57% acetophenone, 21% cumyl alcohol, 10.5% methylstyrene, 1.8% cumene and 9.7% unknown. It was finally determined that the condensed decomposition products served as a lubricant to reduce the co-efficient of friction between the interior walls of the vulcanization tube and the cable insulation thereby reducing frictional drag and preventing oscillation of the cable much as would the saturated steam of a steam cure system. It was found in a dry cure vulcanization system without lubrication, the moving cable tends to momentarily stick at points of contact with the vulcanization apparatus and then after tension on the cable increases to a point at which the static component of the friction force is overcome the cable slides forward until the decreasing tension is less than the Kenetic component of the frictional force at which time the cable momentarily sticks again. This processes is continuously repeated until the insulation is no longer in contact with the interior walls of the curing tube. This is sometimes called a slip-stick phenomenon and is readily seen on a half, semi catenary slant or horizontal vulcanization line where the dry cable insulation touches the bottom of the hot, dry tube inner surface. Interaction of the dry surfaces of these two create oscillation of the cable catenary resulting in vibration which severly limits cable quality and process production rate.